A power capacitor may comprise a plurality of capacitor elements which are electrically connected in parallel and in series. As depicted in FIG. 1a, each such capacitor element comprises two electrode layers 19 and 21 and an intermediate dielectric layer 23. The dielectric layer 23 is thus arranged between the two electrode layers 19 and 21. The dielectric layer 23 may for example comprise a polymer material. The capacitor elements are wound so that a great plurality of turns of the two electrode layers and the dielectric layer are obtained. The capacitor elements can be arranged in a stacked manner forming an active package inside an enclosure or casing of the power capacitor. Inside the enclosure, the active package is immersed in a liquid capable to penetrate the polymer materials.
In FIG. 1a, the capacitor element is shown during a zero-crossing, i.e. when the voltage applied across the two electrodes 19 and 21 is zero. When a voltage is applied to the two electrodes 19 and 21 of the power capacitor, as shown in FIG. 1b, an attractive force, Coulomb force, is generated across the dielectric layer 23 between the two electrodes 19 and 21 because the two electrodes have different electric potential. This attractive force indicated by arrow 17, and also by the arrows between the electrode layers, is simultaneously acting on each turn of the power capacitor. This force will deform the soft dielectric material between the electrodes. The total deformation of the active package is the sum of the deformation across all of the turns of all the capacitor elements. When the voltage crosses the next zero-crossing, the attractive force will become zero as shown in FIG. 1c. When the voltage potential is reversed, charges on the electrodes 19 and 21 will change polarity but the same attractive force will appear once again, as shown in FIG. 1d. This gives rise to a mechanical movement between the electrode layers causing them to oscillate at a frequency twice the frequency of the applied alternating voltage. Since the stack of capacitor elements is immersed in liquid, the vibrations of the capacitor elements are transferred to the capacitor enclosure. The surface vibrations of the enclosure will generate undesirable sound in the form of audible noise.
An example of a power capacitor with noise reduction capabilities is disclosed in EP 0 701 263 B1. The power capacitor comprises at least two capacitor elements which are composed of electrodes separated by one or more dielectrics. The capacitor elements are arranged in a row to form a capacitor package. At least one spring element is arranged between a pair of adjacent capacitor elements in the row or fixed at the outside of the capacitor element at one end of the row. The stiffness of the spring element is adapted such that the external vibrations of the capacitor are reduced. This results in a reduced sound radiation from the capacitor.
WO2014202446 discloses a capacitor device which has a plurality of capacitor elements arranged in rows or stacked. Each capacitor element has two electrodes and a dielectric material arranged between the electrodes. There is between each pair of adjacent capacitor elements a damping element for attenuating vibrations caused by the electrodes.
The spring element and the damping elements in the prior art occupy space inside the enclosure of the power capacitor while not contributing to the capacitance of the power capacitor.